In 1861 he won a major tournament in Vienna, and was sent to represent Austria in the London tournament of 1862. He placed sixth, but Adolf Anderssen, the winner of the event, was impressed by his play. Steinitz immediately challenged the 5th-placed contestant, the Italian player Serafino Dubois, to a match, which Steinitz won (5 wins, 1 draw, 3 losses). This encouraged him to turn professional and he took residence in London. There he established his position as one of the world's top players by a decisive 1863 match victory (7 wins, 1 loss, 2 draws) over the Englishman Joseph Henry Blackburne, who was one of the world's top ten for many decades; he followed this with match victories over Deacon, Augustus Mongredien, Green, and Robey.[3][4] Note: most of Steinitz' competitive chess took the form of matches, as strong tournaments were infrequent in the mid-19th century.[5]

In 1866, Steinitz won a match in London against Adolf Anderssen (8 wins, 6 losses), establishing himself as the best active chess player in the world.[6]. A minority of chess writers date Steinitz as becoming World Chess Champion from that date;[5] there is no evidence that he claimed the title for himself at the time, although he later claimed to have been the champion since his win over Anderssen.[7] It has been suggested that Steinitz could not make such a claim while Paul Morphy was alive.[8] (Morphy had defeated Anderssen by a far wider margin, 8–3, in 1858, but retired from chess soon after and died in 1884).

In the following years Steinitz confirmed his position as "world number one". In tournaments he took: 2nd place at Baden-Baden 1870 behind Anderssen but ahead of Blackburne and Louis Paulsen;[9] 1st at London 1872. His results in match-play were even better: he beat Bird in 1866 (7 wins, 5 losses, 5 draws); crushed Blackburne in 1872 (7 wins, 1 draw); and comfortably beat Johannes Zukertort in 1872 (7 wins, 4 draws, 1 loss; and Zukertort had proved himself one of the elite by beating Anderssen convincingly in 1871).[10] Steinitz was also a good blindfold player: in Dundee 1867 he played six simultaneous blindfold games with the result of three wins and three draws.

All of these victories were achieved in the attack-at-all-costs "Romantic" style exemplified by Anderssen. But in the tournament in Vienna in 1873 Steinitz unveiled a new "positional" style of play which was to become the basis of modern chess (see below).[10] He tied for first place with Blackburne, ahead of Samuel Rosenthal, Paulsen and Henry Bird, and won the play-off against Blackburne.[11] In this tournament he won his last 16 games, and this started a streak of 24 straight wins.[12]

Between 1873 and 1882 Steinitz played no tournaments and only 1 match (a 7-0 win against Blackburne in 1876); instead he concentrated on his work as a chess journalist. Then he played in the 1882 Vienna tournament, which was the strongest chess tournament of all time at that point. Despite a shaky start he took equal 1st place with Szymon Winawer, ahead of James Mason, Zukertort, George Henry Mackenzie, Blackburne, Berthold Englisch, Paulsen and Mikhail Chigorin; and drew the play-off match.[13][12] In 1883 Steinitz took second place in the London tournament behind Zukertort, who made a brilliant start, faded at the end but finished 3 points ahead.[14]

Barely five feet in height, and handicapped by lameness and later by arthritis, Steinitz had a sharp tongue and violent temper.[15] As a result of this, his relations with the London chess community became difficult, so in 1883 he decided to leave England and moved to New York, where he lived for the rest of his life.[12]

Since Steinitz had won the Vienna tournament (1882) and Zukertort had won in London (1883), the chess world thought a match was the only way to decide who was the better player. In 1886, Steinitz and Zukertort played the match, the winner of which was to be recognized as the world champion. Both players were aware of the historical importance of the match.[16] It was played in the U.S. cities New York, Saint Louis and New Orleans. After the five games played in New York, Zukertort led by 4-1, but in the end Steinitz won decisively by 12½–7½ (10 wins, 5 draws, 5 losses). Though not yet officially an American citizen, Steinitz wanted the U.S. flag to be placed next to him (he became a U.S. citizen on November 23, 1888, having resided for five years in New York, and changed his first name from Wilhelm to William).

Steinitz successfully defended his title in 1889 against the Russian Mikhail Chigorin in Havana (10 wins, 1 draw, 6 losses), in 1891 against Isidor Gunsberg (Hungarian-born with British citizenship) in New York (6 wins, 9 draws, 4 losses) and again in 1892 against Chigorin in Havana (10 wins, 5 draws, 8 losses).

After losing the title, Steinitz continued to play in tournaments: he won at New York 1894 and was fifth at Hastings 1895 (winning the first brilliancy prize for his game with Curt von Bardeleben); at Saint Petersburg 1895, a four-players round-robin event with Lasker, Chigorin and Pillsbury, he took a very good second place. Later his results began to decline: 6th in Nuremberg 1896, 5th in Cologne 1898, 10th in London 1899.

Lasker, who took the championship from Steinitz, wrote, "I who vanquished him must see to it that his great achievement, his theories should find justice, and I must avenge the wrongs he suffered."[18] Steinitz's fate, and Lasker's keenness to avoid a similar situation of financial ruin, have been cited among the reasons Lasker fought so hard to keep the world championship title.